Recently, an X-ray image intensifier has been widely applied for the purposes of medical diagnosis, non-destructive examinations and the like. In these applications, an X-ray image obtained by a low-energy X-ray having an X-ray tube voltage of 30 KV (a tube current of 1 mA) or less, or by a high-energy X-ray having an X-ray tube voltage of 30 KV (a tube current of 1 mA) or more, is converted to a visible light image.
As shown in FIGS. 1 and 2, a conventional X-ray image intensifier is basically constructed of an input screen 12, a focusing electrode 13, an anode 14, and an output screen 15, all arranged in a vacuum envelope 11 (hereinafter referred to as an "envelope"). These components are arranged in the order mentioned above, in a direction away from an X-ray source A. The envelope 11 has an input window 11a made of metal, on which an X-ray is incident, a body 11b made of glass for supporting the focusing electrode 13, and an output portion 11c made of optical glass serving as the output screen 15 or as a support for the output screen 15.
The input screen 12, which is provided at a predetermined distance from the input window 11a, functions as a cathode. The input screen 12 is constructed of a curved substrate 12a, for example, an aluminum metal substrate, which has a convex surface formed so as to project toward the X-ray source A. The input screen 12 further includes a phosphor layer 12b for converting an X-ray to visible light, and is formed on the opposite surface of the metal substrate 12a. A transparent conductive film 12c formed on the phosphor layer 12b and a photocathode 12d for converting the visible light from the phosphor layer 12b to electrons, formed on the transparent conductive film 12c also make up the input screen. The transparent conductive film 12c is generally made of indium oxide, ITO (a compound made of indium oxide and titanium oxide) or the like. The transparent conductive film 12c prevents reaction between an alkali halide such as sodium iodide activated cesium iodide phosphor layer 12b and a material constituting the photocathode 12d and provides continuous conductivity on the surface of the phosphor layer 12b.
On the other hand, an anode 14 is disposed at the opposed side to the input screen 12, namely, at the side in which the output screen 15 is disposed (the outer face herein has a structure such that the optical glass substrate supporting output phosphors serves as part of the envelope). The anode 14 is supported by the side in which an envelope output portion 11c is formed. Between the anode 14 and the input screen 12 used as the cathode, a first focusing electrode 13a is provided along the inner wall of the envelope body 11b. Between the first focusing electrode 13a and the output screen 15, a pipe-shape second focusing electrode 13b is provided. The first and the second focusing electrodes 13a and 13b define an electrostatic electron lens system.
In the X-ray image intensifier, an X-ray B radiated from the X-ray source A is transmitted through an object C, reaching the input window 11a. The X-ray image reflected on the input window 11a is converted to an electron image formed on the input face, as will be described later. The electron image is accelerated and focused through the electrostatic electron lens system defined by the first focusing electrode 13a and the second focusing electrode 13b. A tube voltage, which is applied between the input screen 12 as the cathode and the anode 14, e.g., 30 KV of a tube voltage, is divided into two voltages and these voltages are applied to the electrodes, 13a, 13b, respectively. Thereafter, the electron image is converted back into a visible light on the output screen 15. In this way, a visible image can be intensified, for example, 1000 times or more, in proportion to the intensity of the visible light entering the input screen 12.
As shown in an enlarged view of FIG. 2, the input screen of the above-mentioned conventional X-ray image intensifier presents a problem in that the X-ray is scattered, lowering image contrast since the input window 11a and the input screen 12 are separated by a predetermined distance. Hereinbelow, this problem will be explained by way of example of an X-ray image intensifier having an effective input-screen diameter of 4 inches, with reference to FIG. 3.
To obtain data shown in FIG. 3, a tube voltage of 50 KV and a tube current of 1 mA were applied to the X-ray tube. A contrast (%) and a contrast ratio of the X-ray image intensifier are plotted on a vertical axis and a diameter (mm) of a lead circular plate is plotted on the horizontal axis. The contrast herein is indicated in percentage of brightness in the effective input visual field when a lead plate having a predetermined diameter is positioned at the center of the effective input visual field, based on the brightness in the effective input visual field of the X-ray image intensifier when no lead plate is positioned. The contrast ratio is numerically calculated from the contrast values (%).
A curve c of FIG. 3 shows the characteristics of the X-ray image intensifier having the conventional structure shown in FIG. 2. As is apparent from the curve c, as the diameter of the lead circular plate used in measuring contrast becomes smaller than 40 mm, the image contrast significantly reduces. This fact implies that the contrast of a small object image is significantly inferior to that of a large object. From the industrial point of view, this fact leads to a drawback in that it is more difficult to find defects of fine portions in a larger object.
FIG. 4 shows the contrast data obtained from an experiment conducted in the above described manner except that a tube voltage of the X-ray tube is changed to 30 KV, using the same X-ray intensifier. According to the straight line e of FIG. 4, in the same fashion as in the curve c of FIG. 3, as the diameter (mm) of the lead circular plate becomes smaller than 40 mm, the contrast significantly reduces. However, the degree of the image contrast reduction in this case is larger than in the case of FIG. 3.
On the other hand, Jpn. UM Appln. KOKOKU Publication No. 34-20832 and some other publications disclose an X-ray image intensifier comprising an input screen directly formed on the inner surface of an aluminum input-window. However, such an X-ray image intensifier comprising an input screen directly formed on an inner surface of an input window made of aluminum has not yet been put into practical use. If an X-ray image intensifier comprising the input window made of such a thin material is fabricated and then evacuated, the input window will be distorted by the pressure difference between the inside and the outside of the tube. As a consequence of the input screen being distorted, a desired photocathode cannot be obtained and the output image is distorted.